(1) Field of the Invention
The present invention relates to a configuration of an electrostatic capacitance type touch switch designed to detect a touch operation of an operator on the basis of a variation of the electrostatic capacitance between electrodes, and more particularly to a configuration of a touch switch suitable for use in a control panel for devices in the interior of an automobile.
(2) Description of the Related Art
So far, there has been widely known a touch switch designed to, when a portion of a human body comes into contact with or comes close to a conductive electrode member, detect this state for placing a desired device into operation. In the case of such a touch switch, the electrode member functions as an electrostatic capacitance sensor so as to read a variation in impedance (electrostatic capacitance) of the electrode when the human body comes into contact with the electrode, thereby detecting the on/off-state of the touch switch. In general, the electrode member is made of a metallic plate as disclosed, for example, in Japanese Patent Laid-Open No. 2004-22356 (JP 2004-22356).
Moreover, as such an electrostatic capacitance type touch switch, there has been known a type configured by forming electrodes, connection patterns (wiring parts), conductive thin films and others by use of a sputtering technique, a type configured by forming electrodes, connection patterns (wiring parts), conductive thin films and others on a substrate by use of a conductive ink according to a screen printing technique, or other types.
Meanwhile, in the recent years, there has been proposed an idea in which such a touch switch is used in a control panel for devices in the interior of an automobile. Concretely, it is considered to apply it as an on/off switch for an audio device, air conditioner or change-over switch among various modes in an automobile.
With respect to the application to such an automobile control panel, Japanese Patent Laid-Open No. 2006-196395 (JP 2006-196395) discloses a technique in which, when an electrode member is made from an optical non-transparent member, this electrode member is configured as a mesh electrode with a mesh shape in consideration of the operability at night. This configuration allows the transmission of illumination light from a light-emitting device placed in confronting relation to the electrode and enables the compatibility between the illumination light transmission function, although it is made of a non-transparent material, and the function as the electrode member.
In addition, Japanese Patent Laid-Open No. 2007-35334 (JP 2007-35334) discloses a touch switch in which a panel can be shaped freely into a curved surface such as a three-dimensional configuration. Concretely, this document discloses a technique in which a conductive rubber is placed at a backside of a non-conductive panel so that an impedance variation at an touching operation by an operator is transferred through this conductive rubber to an electrode.
However, in the case of a control panel in which a large number of control switches are put on a panel surface, such as an automobile control panel, a configuration using metallic-plate electrodes disclosed in JP 2004-22356 requires that many metallic plates are incorporated into a rear side of a resin-made panel, which leads to a poor assembling workability.
Furthermore, in the case of the technique disclosed in the Patent Document JP 2006-196395, wherein an optical non-transparent mesh electrode is printed onto a rear surface of a resin sheet and the resin sheet is then attached onto a surface of a resin panel, the assembling workability enhances, whereas there is a need to carry out the printing onto a resin sheet and the formation into a three-dimensional configuration, which causes an increase in cost.
Still furthermore, in the case of the technique disclosed in JP 2007-35334, a conductive member (conductive rubber) does not exist on a panel surface and, since the variation of impedance of the conductive rubber is detected through the panel at the touching operation, the detection sensitivity is low and there is possibility that difficulty is experienced in accurately detecting the touching operation. In this case, although it is considered to enhance the detection sensitivity by increasing the electrical output of a detection circuit, an increase in noise occurs and, in a case in which a plurality of touch switches are disposed around it, there is a possibility that an erroneous detection of the touching operation occurs (for example, a judgment is made at the touching operation as a touching operation of an adjacent touch switch).